1. Field of the Invention
This invention relates to foam inhibitor mixtures to inhibit the formation of microfoams in aqueous lacquer and synthetic resin dispersions.
2. Description of the Related Art
Aqueous synthetic resin solutions and dispersions of the type used, for example, as binders for printing inks, wood and metal lacquers, and as coatings for paper and plastics are mostly based on water-soluble alkyd resins, polyacrylates, or polyesters. The alkyd resins used are those with carboxyl functionality, such as oils modified with maleic acid, oil-modified alkyds and oil-free polyesters; in addition to the neutralization component, usually an amine, a co-solvent component, is frequently present. Polyacrylate-based lacquers use copolymeric acrylic acid derivatives which may be combined both with melamine or urea resins and also with polyurethanes. Suitable polyesters are modified unsaturated compounds in emulsion form.
Due to the presence of emulsifiers and salt-forming carboxyl groups, the binder systems present foaming problems during production and application which generally cannot be solved with standard foam inhibitors. Thus, where conventional alkyl polysiloxanes are used in the form of emulsions or in the form of solutions in hydrocarbons, serious surface faults known among those skilled in the art as craters, fisheyes and orange-peel effects are obtained.
On the other hand, the organopolysiloxanes suitable and specifically developed for such applications are comparatively expensive. Accordingly, repeated attempts have been made to replace them completely or in part by inexpensive foam inhibitors.
However, where these inhibitors additionally contain other known defoaming agents, for example, those based on mineral oils or ester oils based on long-chain linear fatty acids, incompatibility with the binder system and exudation on the lacquer surface can occur despite the addition of emulsifiers. Foam inhibitors containing organic solvents, such as low molecular weight glycols, ketones or esters, are attended by the disadvantage that residual foams plainly visible after drying of the lacquers remain in the form of surface foam or microfoam. In addition, such solvents frequently have a low flash point and necessitate additional safety precautions for transport and for handling due to critical MWC (maximum workplace concentration) values and toxic loads. For the reasons explained above, defoaming mixtures such as these are unsuitable for the purposes of the present invention.
In addition, German Application No. 12 57 111 describes foam inhibitors consisting of esters of branched-chain fatty acids and branched-chain monoalcohols. These esters are suitable for use as defoaming agents in the textile, detergent and food industries and also in chemical reactions and in distillation. Their use in aqueous synthetic resin dispersions is not mentioned and it can be shown that, in the absence of other foam inhibitors, the effect of these esters is inadquate.
German Application No. 30 13 391 describes a silicone-free foam inhibitor which consists of a homogeneous mixture of branched-chain C.sub.18 -C.sub.30 primary alcohols, particularly Guerbet alcohols, and silanized silica in a ratio of alcohol to silica of from 100:2 to 100:20. The substantially silicone-free foam inhibitor is primarily intended for incorporation in detergents and cleaners.
German Application No. 31 15 644 teaches that the effect of this foam inhibitor for the particular purpose of incorporation in detergents and cleaners may be enhanced by mixing with waxes, such as paraffin, montan wax or ester waxes, and also by application to a water-soluble salt-like carrier, such as sodium tripolyphosphate. The use of such foam-inhibiting granulates in aqueous synthetic resin dispersions is not mentioned. In fact, the foam inhibitors of the above application No. 31 15 644 would be unsuitable for that purpose due to their high salt content.
In addition, German Application No. 27 45 583 describes foam inhibitors for aqueous polymer dispersions which consist of a mixture of from 10 to 95% of an organic carrier liquid, for example a paraffin oil, from 1 to 5% of a synthetic hard paraffin solidifying at 90.degree. to 100.degree. C., from 0.5 to 2% of the magnesium salt of a fatty acid, from 0.5 to 5% of a hydrophobized silica, and from 2 to 20% of a non-ionic emulsifier. The need to stabilize foam inhibitors such as these by additions of emulsifiers and thickeners (fatty acid magnesium soaps) is a disadvantage because these non-foam-inhibiting additives can lead to imcompatibilities with the aqueous lacquer dispersions, particularly gloss lacquers. In the case of gloss lacquers, so-called microfoams can also lead to serious faults, such as pore and crater formation and loss of gloss. Such microfoams can form, for example, when the lacquer dispersion is applied to the surface to be coated. Unless their formation is suppressed or unless they are destroyed before the lacquer layer hardens, the pores and craters mentioned above are formed.